What's the difference between DEF bones and ORG bones in functionality?
ORG - prefix for original bones from which the full rig is generated

DEF - prefix for bones contributing to deformation

Usually the ORG bones of the metarig are replaced by generated bones, but in this case the clavicle is used from the original rig and contributing to deformations.

This IMHO is a mistake and should be marked right, a copy of that ORG bone should be generated and marked DEF and the ORG bone moved to a hidden layer where rest of the org bones are lurking, ready to be tweaked and a new full rig generated from them again if needed. If both are the same bone, you do deform the mesh while tweaking the metarig and this is not wanted, the mesh should stay intact. If the org bones are in a separate armature (like with rigify), there is no need to have ORG prefixed bones inside the rig at all.